This invention relates to an acoustic signal generating apparatus adopting a PWM (Pulse Wide Modulation) system converting the voltage amplitude of an acoustic signal into a digital pulse width based on digital acoustic data.
Hitherto, there is provided a pulse width modulation circuit for pulse width modulating a supplied carrier signal based on the level of an audio signal and a pulse width modulation amplification circuit for outputting an output pulse of the pulse width modulation circuit and an inversion pulse to a balanced transformerless amplification circuit including a low-pass filter for driving a loudspeaker.
In such a pulse width modulation amplification circuit, a pulse which becomes a carrier signal is output from the pulse width modulation circuit even in a state in which no audio signal is output and thus when power is turned on/off, shock noise caused by appearance/disappearance of the pulse occurs; this is a problem.
Japanese Patent Publication No. 6-196940A proposes an art of circuitry including a pulse width modulation circuit, an oscillator for oscillating output at a higher frequency than the frequency of a carrier signal supplied to the pulse width modulation circuit and synchronized with the carrier signal, a time constant circuit to which a power supply voltage is applied, conversion means for converting the oscillation output of the oscillator into output of a saw tooth wave, a level comparator for making a comparison between the output level of the time constant circuit and the output level from the conversion means, and an exclusive-OR circuit for inputting the output of the level comparator and the output of the pulse width modulation circuit, wherein the output of the exclusive-OR circuit and the output of the pulse width modulation circuit are output to a balanced transformerless amplification circuit.
According to the pulse width modulation amplification circuit described above, output to the balanced transformerless amplification circuit when the power is turned off and that when the power is shut off are placed in phase with each other and generation of shock noise on the load driven by the output of the pulse width modulation circuit and the output of the exclusive-OR circuit is eliminated.
In the pulse width modulation amplification circuit in the related art described above, however, the analog circuits of the conversion means and the time constant circuit need to be provided separately and to cope with the problem, digital signal processing is insufficient; this is a problem. If an audio signal is input, when the phase of the output of the exclusive-OR circuit is in a transition state, distortion occurs in the audio signal output to the load and thus muting must be applied for a given time period in the transition state; this is a problem.
An acoustic signal generating apparatus including oscillation means for generating a carrier signal of a predetermined frequency, PWM signal generator for pulse width modulating the carrier signal based on acoustic data, and a low-pass filter for converting a PWM signal output from the PWM signal generator into an analog acoustic signal without the above-described balanced transformerless amplification circuit cannot adopt the countermeasure based on phase adjustment described above; this is a problem.
If a signal is output from the PWM signal generator in synchronization before and after the pulse width modulating timing based on acoustic data and is fixed high or low in other time periods, the DC level of the low-pass filter fluctuates to the center level before and after the signal output time period and when the signal makes a center to high or low transition, shock noise occurs; this is also a problem.